1. Field of the Invention
The present invention relates to an optical device, an optical system, and an imaging apparatus.
2. Description of the Related Art
In an optical apparatus, such as a camera or the like, an optical diaphragm (or stop), an ND (Neutral Density) filter, or the like is used to adjust an amount of light incident to a lens or the like. The camera is implemented in a mobile phone, a portable terminal, or the like, and a compact optical diaphragm is also used in such a camera, as proposed in Japanese Laid-Open Patent Publication No. 2006-301221, for example.
FIGS. 1A and 1B are diagrams for explaining an example of the diaphragm. A diaphragm 910 includes an aperture 911 at a central part of a plate made of a light blocking material. Light at a peripheral part of the plate is blocked, and the light passes through the central part where the aperture 911 is formed. FIG. 1A is a top view of the diaphragm 910, and FIG. 1B illustrates a light transmittance along a one-dot chain line 1A-1B in FIG. 1A.
On the other hand, in the optical diaphragm 910, diffraction of light generated in a periphery of the aperture 911 is becoming no longer negligible, and it is becoming more difficult to improve the resolution. In other words, as a number of pixels of the camera increases, there are demands for a compact optical diaphragm that does not deteriorate the resolution.
As an example of an optical device having the optical diaphragm that satisfies such demands, there is a known apodized filter having a configuration in which the transmittance of visible light is high at the central part, and the transmittance decreases from the central part towards the peripheral part, as illustrated in FIGS. 2A and 2B. FIGS. 2A and 2B are diagrams for explaining an example of the apodized filter. FIG. 2A is a top view of an apodized filter 920 having an aperture 921 at the central part thereof, and FIG. 2B illustrates the light transmittance along a one-dot chain line 2A-2B in FIG. 2A.
Related art further includes a hard coat film proposed in Japanese Laid-Open Patent Publication No. 2011-227488, for example.
In the diaphragm 910 described above, the aperture 911 is air, and there is no deterioration caused by ultraviolet (UV) light. On the other hand, in the apodized filter 920 described above, a material through which visible light is transmitted exists at the aperture 921, and this material may be exposed to the ultraviolet light. When the material at the aperture 921 is exposed to the ultraviolet light, yellowing or cracks may be generated at the aperture 921, to thereby reduce the visual light transmittance of the aperture 921. In other words, an apodized filter having a resistance to the ultraviolet light at the aperture 921 is also desired.